Breakthroughs are coming thick and fast – or should that be thin and fast – in the field of nanoscale lasers. It wasn’t even a month ago that we reported on the development of a laser emitting 'metal-semiconductor-metal sandwich', made up of a semiconductor as thin as 80 nanometers laying between 20-nanometer dielectric layers. But now researchers at the University of California, Berkeley, have reached a new milestone in laser physics by creating the world's smallest semiconductor laser, capable of generating visible light in a space smaller than a single protein molecule.

The researchers say their breakthrough marks a major advance toward applications in the biomedical, communications and computing fields. These include the development of nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power, but with greater power efficiency.

The problem

While it was traditionally accepted that an electromagnetic wave - including laser light – couldn’t be focused beyond the size of half its wavelength, research teams around the world discovered it was possible to compress light to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as surface plasmons.

Since this discovery scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical excitations. However, the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the build-up of the electromagnetic field necessary for lasing.

The solution

To overcome this problem the researchers took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire - 1,000 times thinner than a human hair - with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished.

The UC Berkeley researchers used semiconductor materials and fabrication technologies that are commonly employed in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the coherent state that is a key characteristic of a laser.

"What is particularly exciting about the plasmonic lasers we demonstrated here is that they are solid state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale," said Volker Sorger, a PhD student and study co-lead author.

Scientists hope to eventually shrink light to the size of an electron's wavelength, which is about a nanometer, or one-billionth of a meter, so that the two can work together on equal footing.

The researchers' work is detailed in the paper, “Plasmon lasers at deep subwavelength scale”, which appears in the journal Nature.

Schematic of light being compressed and sustained in the 5 nanometer gap (left) and an electron microscope image of the hybrid design shown in the schematic (Image: Xiang Zhang Lab, UC Berkeley)

Light from a single plasmon laser emanating from the optical setup used by UC Berkeley researchers (Photo: Xiang Zhang Lab, UC Berkeley)

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